A Robust Bio-IA With Digitally Controlled DC-Servo Loop and Improved Pseudo-Resistor

Bio-signal instrumentation amplifier (Bio-IA) is one of the most important circuit in the vital sign monitoring system. However, the performance of the Bio-IA using traditional pseudo-resistor (PR) is always very sensitive to process, power voltage and temperature (PVT). This brief proposed a digitally controlled dc-servo loop (DCDSL) and an improved PR to improve the robustness of Bio-IA. The proposed DCDSL replaces the integrator-based DSL and eliminates the PR in the traditional DSL, which can improve the response speed as well as the noise performance. The presented Bio-IA was implemented in a standard 0.18 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS process, occupying an active area of <inline-formula> <tex-math notation="LaTeX">$0.474\times 0.424$ </tex-math></inline-formula> mm². The measured results shows that the Bio-IA can operate in the temperature range of −20°C~80°C. When the bias current is 500 fA, the equivalent resistance of the improved PR can be up to 330 <inline-formula> <tex-math notation="LaTeX">$\text{G}{\boldsymbol{\Omega }}$ </tex-math></inline-formula>. The proposed Bio-IA can calibrate the dc offset as large as ±300 mV with a response time less than 200 ms. When the dc offset increases from 0 to 300 mV, its input-referred noise varies from 0.67 <inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>Vrms to 1.49 <inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>Vrms. With 1.2 V supply voltage, the midband gain of the Bio-IA is 40 dB.